The present invention relates to a metal-insulator semiconductor field-effect transistor (MISFET).
Field-effect transistors (FET) are semiconductor components having three connections, namely a source connection, a drain connection and a gate electrode. The current flowing between the source and the drain is controlled by applying an electric field to the gate electrode in a direction perpendicular to the current flow. The field results in enhancement or depletion of the carriers and, thus, changes the size of a channel which conducts current in the semiconductor. When no control voltage is being fed to the gate electrode, this channel is either open (normally on) or closed (normally off).
There are three basic types of FETs, including:
MESFET (MES=metal semiconductor), in which the gate electrode is arranged directly on the semiconductor; PA1 JFET (J=junction), in which a p-n junction is arranged between the gate electrode and the conductive channel; and PA1 MISFET (MIS=metal-insulator semiconductor), in which an insulating layer is arranged between the gate electrode and the semiconductor.
MOSFET (MOS=metal-oxide semiconductor) constitutes a special type of the last-mentioned MISFET type. In the case of the MOSFET, the insulating layer consists of an oxide of the semiconductor. FET and, in particular, MISFET are manufactured primarily with silicon (Si). However, silicon is no longer functional at temperatures over 200.degree. C. due to its relatively small energy gap between the valence band and the conduction band of 1.1 eV.
Silicon carbide (SiC) is known as a material for electronic high-temperature components. Its electronic properties make it possible for SiC to be used at temperatures of up to about 1300 K. The special advantages the semiconductor material SiC has over the elementary semiconductor Si are, besides its larger band gap of between 2.2 eV and 3.4 eV, in accordance with the polytype used, compared to 1.1 eV for silicon, its breakdown dielectric strength of about 4.times.10.sup.6 V/cm, which is higher by a factor of 13, the three times greater thermal conductivity, and the saturation drift velocity that is greater by a factor of 2.3.
A MOSFET having a SiC basis is known, in which a thin p-conductive SiC film is deposited on a p-conductive 6H-SiC substrate. Two more heavily doped n.sup.+ regions are arranged as a source and, in between, an n.sup.+ region as a drain on the surface of the n-SiC film. These n.sup.+ regions are contacted with corresponding electrodes consisting of TaSi.sub.2. A gate electrode of polycrystalline silicon is arranged in each case between the two source regions and the drain region. Disposed in each case between these gate electrodes and the n-SiC film is an oxide layer of SiO.sub.2, which also insulates the corresponding gate electrode from the other electrodes. This oxide layer can be produced relatively simply through the thermal oxidation of the SiC layer (J. Appl. Phys., Vol. 64, No. 4, Aug. 15, 1988, pp. 2168-2177). To produce an inversion layer in the case of such a SiC MOSFET, the gate electrode must be fed higher gate voltages than in the case of a comparable Si MOSFET, due to the larger energy gap of SiC compared to Si. However, the breakdown dielectric strength of SiO.sub.2 lies merely at about 3.times.10.sup.6 V/cm. Given a gate voltage of, for example, 50 V, one must select such a thick SiO.sub.2 layer that the channel can no longer be controlled, because of the smaller capacitance associated with such a layer. A further disadvantage of SiO.sub.2 is its increasing ionic conductivity at higher temperatures. In the high-temperature range above 200.degree. C., leakage currents arise, therefore, between the gate and drain in the case of the described MOS structure, which adversely affect the functioning of the MOSFET.
Generally, all semiconductors having a correspondingly greater energy gap than Si come under consideration for use in the high-temperature range. Examples are: gallium arsenide (GaAs) having an energy gap of 1.4 eV, gallium phosphide (GaP) having 2.3 eV, gallium nitride (GaN) having 3.4 eV., aluminum phosphide (AlP) having 3.12 eV, aluminum nitride (AlN) having 6.2 eV, or boron nitride (BN) having 7.5 eV (in each case at 300 K.). Contrary to SiC, these materials do not form any naturally occurring oxide, which could be produced through thermal oxidation. A SiO.sub.2 insulating layer for a MISFET on the basis of these semiconductors would, therefore, have to be deposited, for example, using sputtering techniques. The insulating properties of a SiO.sub.2 layer produced in this manner, however, are generally even worse than those of a thermally grown SiO.sub.2 layer.